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In 2012, the landing ellipse for the Curiosity Lander was only 4 miles wide by 12 miles long,an area more than 200 times smaller than Viking's.This allowed NASA to target a specific spot in Gale Crater,a previously un-landable area of high scientific interest.While we ultimately strive for accuracy,precision reflects our certainty of reliably achieving it.With these two principles in mind,we can shoot for the stars and be confident of hitting them every time.

Weighing all the possibilities,the computer spits out the potential area of impact in the form of a landing ellipse.In 1976, the landing ellipse for the Mars Viking Lander was 62 x 174 miles,nearly the area of New Jersey.With such a limitation,NASA had to ignore many interesting but risky landing areas.Since then, new information about the Martian atmosphere,improved spacecraft technology,and more powerful computer simulations have drastically reduced uncertainty.

Predicting where they will land requires extensive calculations fed by measurements that don't always have a precise answer.How does the Martian atmosphere's density change at different elevations?What angle will the probe hit the atmosphere at?What will be the speed of the probe upon entry?Computer simulators run thousands of different landing scenarios,mixing and matching values for all of the variables.

Factories and labs increase precision through better equipment and more detailed procedures.These improvements can be expensive,so managers must decide what the acceptable uncertainty for each project is.However, investments in precision can take us beyond what was previously possible, even as far as Mars.It may surprise you that NASA does not know exactly where their probes are going to touch down on another planet.

The story of the stolen crossbow was one of precision without accuracy.William got the same wrong result each time he fired.The variation with the shaky hand was one of accuracy without precision.William's bolts clustered around the correct result,but without certainty of a bullseye for any given shot.You can probably get away with low accuracy or low precision in everyday tasks.But engineers and researchers often require accuracy on microscopic levels with a high certainty of being right every time.

The distinction between the two is actually critical for many scientific endeavours.Accuracy involves how close you come to the correct result.Your accuracy improves with tools that are calibrated correctly and that you're well-trained on.Precision, on the other hand,is how consistently you can get that result using the same method.Your precision improves with more finely incremented tools that require less estimation.

His practice shots still cluster around the apple but in a random pattern.Occasionally, he hits the apple,but with the wobble,there is no guarantee of a bullseye.He must settle his nervous hand and restore the certainty in his aim to save his son.At the heart of these variations are two terms often used interchangeably:accuracy and precision.

However, the borrowed crossbow isn't adjusted perfectly,and William finds that his practice shots cluster in a tight spread beneath the bullseye.Fortunately, he has time to correct for it before it's too late.Variation two:William begins to doubt his skills in the long hours before the challenge and his hand develops a tremor.

As the story goes, the legendary marksman William Tell was forced into a cruel challenge by a corrupt lord. William's son was to be executed unless William could shoot an apple off his head.William succeeded, but let's imagine two variations on the tale.In the first variation,the lord hires a bandit to steal William's trusty crossbow,so he is forced to borrow an inferior one from a peasant.

Perhaps we'll find new exotic forms of matter that'll force us to revisit the laws of thermodynamics.Or maybe there's perpetual motion on tiny quantum scales.What we can be reasonably sure about is that we'll never stop looking.For now, the one thing that seems truly perpetual is our search.

That heat is energy escaping,and it would keep leeching out,reducing the energy available to move the system itself until the machine inevitably stopped.So far, these two laws of thermodynamics have stymied every idea for perpetual motion and the dreams of perfectly efficient energy generation they imply.Yet it's hard to conclusively say we'll never discover a perpetual motion machine because there's still so much we don't understand about the universe.

There are ones that seem to keep going,but in reality, they invariably turn out to be drawing energy from some external source.Even if engineers could somehow design a machine that didn't violate the first law of thermodynamics,it still wouldn't work in the real world because of the second law.The second law of thermodynamics tells us that energy tends to spread out through processes like friction.Any real machine would have moving parts or interactions with air or liquid molecules that would generate tiny amounts of friction and heat,even in a vacuum.

Then there are versions with magnets,like this set of ramps.The ball is supposed to be pulled upwards by the magnet at the top,fall back down through the hole,and repeat the cycle.This one fails because like the self-watering pot,the magnet would simply hold the ball at the top.Even if it somehow did keep moving,the magnet's strength would degrade over time and eventually stop working.For each of these machines to keep moving,they'd have to create some extra energy to nudge the system past its stopping point,breaking the first law of thermodynamics.

With a low center of mass,the wheel just swings back and forth like a pendulum,then stops.What about a different approach?In the 17th century, Robert Boyle came up with an idea for a self-watering pot.He theorized that capillary action,the attraction between liquids and surfaces that pulls water through thin tubes,might keep the water cycling around the bowl.But if the capillary action is strong enough to overcome gravity and draw the water up,it would also prevent it from falling back into the bowl.

There wouldn't be any left over to power a car or charge a phone.But what if you just wanted the machine to keep itself moving?Inventors have proposed plenty of ideas.Several of these have been variations on Bhaskara's over-balanced wheel with rolling balls or weights on swinging arms.None of them work.The moving parts that make one side of the wheel heavier also shift its center of mass downward below the axle.

There's just one problem.They don't work.Ideas for perpetual motion machines all violate one or more fundamental laws of thermodynamics,the branch of physics that describes the relationship between different forms of energy.The first law of thermodynamics says that energy can't be created or destroyed.You can't get out more energy than you put in.That rules out a useful perpetual motion machine right away because a machine could only ever produce as much energy as it consumed.

Imagine a windmill that produced the breeze it needed to keep rotating.Or a lightbulb whose glow provided its own electricity.These devices have captured many inventors' imaginations because they could transform our relationship with energy.For example, if you could build a perpetual motion machine that included humans as part of its perfectly efficient system,it could sustain life indefinitely.

Around 1159 A.D.,a mathematician called Bhaskara the Learned sketched a design for a wheel containing curved reservoirs of mercury.He reasoned that as the wheels spun,the mercury would flow to the bottom of each reservoir,leaving one side of the wheel perpetually heavier than the other.The imbalance would keep the wheel turning forever.Bhaskara's drawing was one of the earliest designs for a perpetual motion machine,a device that can do work indefinitely without any external energy source.